The work function of a Ni fully silicide gate or other metal can be tuned by adding alloy element to Ni target for Ni sputter deposition. The work function being the minimum energy needed to remove an electron from the Fermi level in a metal to a point at infinite distance away outside the surface. The work function is generally about half the ionization energy of a free atom of the metal.
The alloy element in Ni silicide is concentrated in the surface after silicide formation. However, the work function of a fully silicide gate is determined at the silicide/dielectric interface or the bottom of the silicide. Therefore, alloy element has little influence on the work function. Though alloy element can be redistributed uniformly in the silicide by high temperature annealing as known in the art, there is not effective way to drive alloy element to the bottom.
In order to enhance the alloy element effects, larger amount of alloy element is added into Ni target. However, when larger amounts of alloy element are added into Ni target, several issues occur, such as a phase separation of the microstructure in the target, which will impact sputter uniformity as well as increasing the possibility of thermal-stress cracking caused by thermal expansion misfits among brittle silicide phases. Also, the maximum amount of element is limited by Ni-alloy mutual solubility. Hence, the work function of a Ni-alloy silicide gate is thus limited.
Often, a different type of metal is desired or a different amount of silicidation is desired in order to create varying work functions dependent upon the device and its characteristics. Thus, there is a need for a silicided structure in which characteristics may be tuned or optimized for a particular application.
In order to obviate the deficiencies of the prior art, the present subject matter drives the alloy element from the surface to the silicide bottom (or interface) and achieve wide range of alloy concentration and obtain high work function for PMOS or low work function for NMOS. With more alloy element at silicide/dielectric interface, the work function limitation of Ni-alloy silicide for NMOS and PMOS can be surpassed.
It is an object of the present subject matter to present a semiconductor device with a controlled work function. The semiconductor device includes a semiconductor substrate; a gate dielectric over the substrate; and a first metal silicide layer over the gate dielectric. The first metal silicide has a first phase and comprises at least one alloy element. The semiconductor device also includes a second metal silicide layer over the first metal silicide layer. The second metal silicide comprises the same metal as the first metal silicide layer but with a different second phase and also includes at least one alloy element.
It is another object of the present subject matter to present method for adjusting the work function of a FUSI gate. The method including the steps of providing a semiconductor substrate; providing a gate dielectric over the substrate; and depositing a first and second metal silicide layer respectively over the gate dielectric. The first and second metal silicide layers having an alloy element and each having a different phase. The amount of alloy in the first and second metal silicide layers is selected to thereby adjust the work function of the FUSI gate.
These and many other objects and advantages of the present subject matter will be readily apparent to one skilled in the art to which the subject matter pertains form a perusal of the claims, the appended drawings and the following detailed description of the preferred embodiments.